feat(ollama): periodically refresh models (#2805)

For self-hosted providers like Ollama (or vLLM), the backing server is
running a set of models. That server should be treated as the source of
truth and the Stack registry should just be a cache for those models. Of
course, in production environments, you may not want this (because you
know what model you are running statically) hence there's a config
boolean to control this behavior.

_This is part of a series of PRs aimed at removing the requirement of
needing to set `INFERENCE_MODEL` env variables for running Llama Stack
server._

## Test Plan

Copy and modify the starter.yaml template / config and enable
`refresh_models: true, refresh_models_interval: 10` for the ollama
provider. Then, run:

```
LLAMA_STACK_LOGGING=all=debug \
  ENABLE_OLLAMA=ollama uv run llama stack run --image-type venv /tmp/starter.yaml
```

See a gargantuan amount of logs, but verify that the provider is
periodically refreshing models. Stop and prune a model from ollama
server, restart the server. Verify that the model goes away when I call
`uv run llama-stack-client models list`

docs/source/providers/inference/remote_ollama.md

@@ -9,6 +9,8 @@ Ollama inference provider for running local models through the Ollama runtime.
 | Field | Type | Required | Default | Description |
 |-------|------|----------|---------|-------------|
 | `url` | `<class 'str'>` | No | http://localhost:11434 |  |
+| `refresh_models` | `<class 'bool'>` | No | False | refresh and re-register models periodically |
+| `refresh_models_interval` | `<class 'int'>` | No | 300 | interval in seconds to refresh models |
 
 ## Sample Configuration